It is known that a latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. Such dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be produced (1) by providing a photoconductive layer or surface, e.g., photopolymerized, with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy, or (2) by applying a uniform electrostatic charge on an imagewise exposed photopolymer surface. Other methods are known for forming latent electrostatic images. For example, one method is providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. Useful liquid toners comprise a thermoplastic resin and dispersant nonpolar liquid. Generally a suitable colorant is present such as a dye or pigment. The colored toner particles are dispersed in the nonpolar liquid which generally has a high-volume resistivity in excess of 10.sup.9 ohm centimeters, a low dielectric constant below 3.0 and a high vapor pressure. The toner particles are less than 10 .mu.m average by area size. After the latent electrostatic image has been formed, the image is developed by the colored toner particles dispersed in said dispersant nonpolar liquid and the image may subsequently be transferred to a carrier sheet.
Since the formation of proper images depends on the differences of the charge between the liquid developer and the latent electrostatic image to be developed, it has been found desirable to add a charge director compound and preferably adjuvants, e.g., polyhydroxy compounds, aminoalcohols, polybutylene succinimide, an aromatic hydrocarbon, etc., to the liquid developer comprising the thermoplastic resin, dispersant nonpolar liquid, and preferably a colorant dispersed in the thermoplastic resin. Other adjuvants, e.g., metallic soaps such as aluminum tristearate, etc., and metal salts can also be dispersed in the resin. Such liquid developers provide images of good resolution. Controlling the magnitude and polarity of liquid electrostatic developers is a necessary but difficult task. For example, the charging and image quality of such liquid developers are particularly pigment dependent. In addition, many of the previously known liquid electrostatic developers are negative-working, i.e., they are attracted to surfaces that are positively charged.
There is a need for a positive-working liquid electrostatic developer having good quality solid area coverage, resolution and toning of fine details which is prepared from dispersant nonpolar liquid, ionic or zwitterionic charge director, and a thermoplastic resin preferably having dispersed therein a colorant.